Fiber optic-based telecommunication networks are capable of transmitting data over several tens of kilometers without signal amplification. Transmitting data over distances greater than 100 kilometers requires that the signal be amplified. Currently, the two most popular optical amplifiers are erbium doped fiber amplifiers (EDFAs) and optical fiber amplifiers utilizing the Raman effect.
Regardless of the type of optical amplifier used in an optical network, the network's signal capacity is limited by the amplifier's spectral gain width as well as any associated gain non-uniformities. Gain non-uniformities within the utilized gain spectrum result in a non-uniform bit error rate across the network channels. Accordingly, a conventional network utilizing wavelength division multiplexing (WDM) technology requires gain flatness of less then±1.0 dB. As the demand for data increases, the required data rate increases, as does the number of required wavelength channels. Since the gain bandwidth of Raman amplifiers is not intrinsically limited, optical amplifiers based on the Raman effect have recently become the focus for commercial development.
The wavelength at which a Raman amplifier provides gain is determined by the wavelength of its pump laser. Therefore, through appropriate choice of pump wavelength, Raman amplifiers can provide signal amplification for any wavelength channel within the transparency range of an optical fiber. Broad bandwidth Raman amplifiers have become important for today's high speed communication systems. In order to achieve broad bandwidth, multiple pump wavelengths are normally used. However, the competing interests in providing both broad pump bandwidths and high pump powers (or the order of 1 W) have led to a number of problems. For example, pump-pump interactions cause the short wavelength pumps to transfer energy to the long wavelength pumps, increasing the power requirements on the short wavelength sources and causing complications in the gain flattening algorithms. Additionally, pump-pump interactions cause a “tilt” in the noise figure of the amplifier, increasing the noise figure of some channels. Pump-pump four wave mixing (FWM) can created new frequencies that lie directly over signal channels, causing impairments to a transmitted data stream.
These problems have been addressed in the past by using a wavelength multiplexed pump source that outputs a high power, single beam of relatively large bandwidth. U.S. Pat. No. 6,456,756 issued to R. D. Mead et al. on Sep. 24, 2002 discloses the formation of a Raman pump source utilizing one or more multi-gain element arrays multiplexed together within a single external resonator cavity. Interposed between the array and the resonator cavity output coupler are a collimating element and a diffraction grating. This combination forces each emitter within the array to lase at a distinct wavelength. Multiple arrays of differing center wavelength may then be packaged together to provide a broad bandwidth source.
The problems associated with prior art arrangements can be mitigated by separating the pump wavelengths temporally, so that they no longer overlap in time. Because Raman and FWM are very fast (femtosecond) processes, if the wavelengths do not overlap in time, pump-pump interactions cannot occur.
Such a pump can be realized by time division multiplexing pulsed pump sources together, or by continuously sweeping the wavelength of a single source. Because of the time dependent nature of the pump, such Raman amplifiers must be operated in a counter-pumped geometry, where the optical data signals propagate in the opposite direction of the pump, allowing the data signals to “see” an averaged gain, rather than a gain that fluctuates as a function of time. Estimates show that in order to perform swept wavelength pumping without impairments to the signals, the pump repetition rate must be greater than 100 kHz. Such a sweep rate is fast enough to ensure that the counter propagating signals see a constant, averaged gain.
A remaining problem in the prior art is the difficult in creating a pump source that has a sweep repetition rate greater than 100 kHz while also sweeping through a broad bandwidth (e.g., tens of nanometers).